Vice President Biden heads into Thursday's debate with Rep. Paul Ryan (R-Wis.) with opinion stacked against him. According to a new Pew Research Center poll, more voters view the vice president unfavorably than favorably by a 51 percent to 39 percent margin.

Opinions on GOP vice presidential nominee Ryan are more evenly divided, with 44 percent of voters viewing him favorably and 40 percent viewing him unfavorably.

Biden's image has deteriorated significantly since his 2008 vice presidential debate with then-Alaska Gov. Sarah Palin. At that time, he was viewed favorably by 53 percent of voters and unfavorably by 31 percent. But Palin was also more popular at the time than Ryan is now, with 51 percent of voters viewing her favorably and 40 percent unfavorably.

Of course, President Obama has consistently been viewed as more likable than challenger Mitt Romney, but that didn't help in last week's presidential debate.